Channel 4 began this year in unusually bullish public form. On the programming front, chief creative officer Jay Hunt said 2011 was the year "Channel 4 got its balls back". Meanwhile, on the matter of money, chief executive David Abraham defied market expectations, talking up C4's ratings and revenue performance.

It was the first time, he said, that the company had topped £1bn in total revenues and, although it would mean spending some reserves and posting a small annual loss at the end of 2012, he announced an £80m increase in spending on UK-originated programmes. And, as to the future, he also went out of his way to offer reassurance that the post-Big Brother Channel 4 was as popular with advertisers as it had ever been, complimenting new sales director Jonathan Allan on concluding "very successful deals with all the major agencies".

It was a surprise, therefore, to read on the industry website Brand Republic just a fortnight later that Channel 4 had written to those same agencies warning them that the channel would almost certainly fail to deliver on its advertising deals this month. In other words, that it would deliver significantly fewer ratings than anticipated in the original deals. Channel 4 says this unusual move is part of a new strategy to be more open with clients about performance. Nevertheless market suspicions are brewing to the effect that Channel 4 might be in rather more of a pickle than it has so far let on.

In theory, under-delivery of ratings in one month really shouldn't matter too much because, provided the shortfall is made up over the rest of the year, everyone will go away happy enough. And even if there is ultimately any notable annual shortfall – or "deal debt" in the jargon – that would normally be taken account of in the next year's trading deal. But underperformance worthy of a note to the clients is perhaps suggestive of something more serious. And when you look at the numbers you can begin to see the scale of the challenge that Channel 4 is facing.

Although Abraham was able to present its overall 2011 ratings performance in positive terms, the fact is that the main channel's ratings did decline and, somewhat hidden from view, its share of the all-important 16-34 demographic fell by 5%.

So the market will have been looking to Channel 4 for evidence that it is on top of the problem. And while it is, of course, early days – 2012 is less than two months old – and over short time-spans individual programmes can have a disproportionate impact on outcomes (in other words, Big Fat Gypsy Weddings only starts its run tomorrow night!), the early signs are not encouraging.

According to industry figures for the main Channel 4 network, which includes C4 +1, the year-on-year decline in all viewing is 13.5%, amongst ABC1 adults it is 15.8%, and for the commercially critical 16-34s, 20%. What is more, early indications for February are worse – all viewing down 17% and peak time down by 24%.

Of course C4 will hope to trade its way out of the problem as the year goes on and, if you look at its performance at the end of 2011 (which bucked the trend by actually beating 2010), and throw in some of the successes it has seen so far this year, the game is far from over.

However, whichever way you look at it, the next advertising deals season will undoubtedly see renewed pressure on Channel 4's premium prices (it nets nearly 18% of all TV ad revenues while delivering only 10% of commercial viewing); and, if ratings performance doesn't improve significantly, lots of those lovely advertising agencies wanting their own back – their very own "mission with mischief", if you like.